A double damping flywheel comprises two coaxial inertia flywheels, i.e. a primary flywheel which is fitted fixed at the end of the crankshaft, and a secondary flywheel which forms a reaction plate of the clutch. The secondary flywheel is centred and guided in rotation on the primary flywheel by means of a bearing.
A torsion damper with springs and friction means is fitted between the two flywheels, in order to transmit rotational torque from one flywheel to the other, whilst absorbing and damping the vibrations and impacts.
The ends of the springs are supported on stops which are integral with the flywheels, and are arranged such that any rotation or oscillation of one flywheel relative to the other gives rise to compression of the springs of the damper in one direction or the other, and is countered by this compression.
The axial frequency of the primary flywheel itself must be low, for example lower than 50 Hz, so that it does not begin to resonate when the engine is running at low speed. For this purpose it is known to use a flexible primary flywheel comprising at least two stacked flexible discs, the inner peripheries of which are designed to be secured to a drive shaft, and the outer peripheries of which are designed to be secured to an inertia mass, each flexible disc comprising two flat parallel parts which are offset axially and are connected to one another by a curved part. The flexible discs have identical profiles, and are superimposed on one another and nested in one another. A structure of this type is known in particular from document EP 0 984 184.
The axial vibrations of the flexible flywheel generate deformation of the aforementioned discs. Since the curved parts of the discs are nested in one another without play, the discs cannot be displaced radially relative to one another, which gives rise to buttressing of the discs at the level of their curved part, and thus to stiffening of the assembly. This tends to reduce the efficiency of the flexible flywheel.